The present invention relates to a control system for an automatic transmission for motor vehicles of the type having a mechanical planetary gear train with hydraulically actuated controls for selective engagement of the transmission elements, which control system automatically disconnects the forward gear clutch of the transmission at certain operating states of the vehicle, e.g. when the accelerator pedal is released and the vehicle is stationary.
A control system of the general type has been disclosed in commonly owned U.S. patent application Ser. No. 061,058, filed July 26, 1979. A manual control valve delivers a first pressurized hydraulic fluid to actuate a control element which is selected by the gear select lever of the transmission when the transmission is in one of the forward gears. A main control valve is arranged in the fluid line between the manual control valve and the gear shift element and operates as a function of the accelerator pedal position and the speed of the vehicle. The main control valve includes a piston which is displaceable to control the delivery of the first pressurized fluid to the selected gear shift element. The piston is acted upon by a spring, an accelerator pedal position dependent fluid pressure, and a vehicle speed dependent fluid pressure.
In hydrodynamic-mechanical automobile transmissions, when the vehicle is stationary and idling engine load continues to be transmitted through the transmission to the power train and the wheels. This normally results in vehicle creep and also acts to transmit engine vibrations through the power train. In the aforementioned U.S. patent application Ser. No. 061,058, a control system is shown and described which under certain operating conditions of the vehicle, for example, when the vehicle is stationary and idling, interrupts the mechanical connection between the engine and the wheels so that creeping does not occur. At the same time, vibration originating at the motor is not transmitted through the drive train and thus vehicle oscillations are suppressed.
More particularly, the main control valve, which is interposed in the delivery line between the manual control valve and selected gear shift element, interrupts the supply of pressurized fluid to the gear shift element when the accelerator pedal is released and vents the fluid in the forward clutch, thereby to disengage the forward clutch when the vehicle is stationary and the accelerator pedal is released. The forward clutch remains disengaged until the accelerator pedal is at least partially depressed or until detection of other conditions, for example, a change in the position of the gear select lever (e.g., when the lever is moved to the position "1" or "2" instead of automatic forward drive "D") or the detection of a predetermined vehicle speed.
More particularly, in the control system described in the aforementioned U.S. patent application Ser. No. 061,058, the main control valve is connected to a second fluid line which delivers an accelerator pedal position dependent pressure to act on the piston of the valve, thereby to selectively engage or disengage the forward gear clutch. A purging control valve is connected to this fluid line and is actuated by the accelerator pedal to vent the fluid line when the accelerator pedal is released. On one embodiment, the main control valve is controlled as a function only of the position of the accelerator pedal, that is, the accelerator dependent pressure in the fluid line. In this embodiment the forward gear control or selected gear shift element disengages at all times when the accelerator pedal is released regardless of the speed of the vehicle.
In a preferred embodiment, the piston of the main control valve is also controlled as a function of the vehicle speed. A vehicle speed-dependent pressure is supplied to the main control valve and acts cumulatively with the accelerator pedal dependent pressure against the force of the return spring. In this embodiment the selected gear shift element disengages only when both the accelerator pedal is released and the vehicle is moving below a pre-set speed, for example, at idle or very low road speeds.